Electrical issue
 

In our Trucks, with the ignition off and a "probe" clipped to a position on the fuse panel that should be "hot" only when the key is in the "On" position and the probe touching Ground ... should the probe light EVER light up??? Remember, the key is "Off".

Re: Electrical issue
 

Yes. There can be "always-on" fuses and fuses that are turned on by the ignition switch. Possible examples: Headlight switch, horn, lighter, hazard, dome, brake lights...... Just see what operates with switch off to confirm.
Many fuse panels have a split hot bar. One switched, one not.
Some of the items, I list, have a direct hot wire to them, like the headlight switch. That switch can have it's own fuse. The GMC fuse philosophy could have changed with the years. The wiring diagrams are VERY hard to read in the area of fuse panel. Can't exactly see what is a switched fuse and what is not. So a little time spent on testing will tell the tale.
You did not mention what fuses you are talking about and if you tested each and every one.
Also, advise if you have just done some wiring.